1. Field of The Invention
This invention relates to wireless communication systems and, more particularly, to multiple sub-carrier communication systems.
2. Description of Related Art
Soft handover is a well known technique in direct sequence code division multiple access (CDMA) systems where it is used to increase the system capacity and reliability. In a hard handover, a mobile unit selects the base station producing the strongest signals among a plurality of base stations. Because of the fading statistics for the transmission path between the mobile unit and the base station, however, signal fading occurs. As such, there is often no single base station producing the strongest signals all of the time. In cases where no base station transmits signals having a dominant power level all the time, soft handover is a technique where a mobile unit is simultaneously communicating with multiple base stations. In doing so, soft handover increases the capacity and reliability of the communication system because soft handover provides diversity. Diversity is a method of using independent fading signals received on several transmission paths all carrying the same message to improve the reliability of the transmission. Proper combination of the signals from these transmission paths yields a resultant signal with reduced fading and improved reliability. Accordingly, instead of switching to a single base station (hard handover), the mobile unit maintains communication links with multiple base stations, so a communication outage can only occur if all the communication links fail simultaneously. When using a soft handover, a mobile unit can typically transmit signals with less power to maintain a given link quality because of the increased reliability that diversity provides. Consequently, the mobile unit creates less interference, thereby increasing the capacity of the network.
Multiple sub-carrier modulation systems typically involve a data signal of successive symbols being split into several lower rate signals (multiple sub-carrier signal), such that the symbol time becomes large compared to the expected multipath delay spread. One such multiple sub-carrier modulation scheme is called orthogonal frequency division multiplexing (OFDM). OFDM is a block-oriented modulation scheme that maps data symbols onto N orthogonal sub-carriers separated by a distance of 1/T, where T is the block period. Multiple sub-carrier transmission systems use OFDM modulation to send data bits in parallel over multiple, adjacent sub-carriers (also called tones or bins). The term "sub-carrier" is used because although the data signal is modulated onto carriers of different frequencies, each "carrier" only carries a fraction of the data signal.
An important advantage of multiple sub-carrier transmission is that inter-symbol interference due to signal dispersion (or delay spread) in the transmission channel can be reduced or even eliminated by inserting a guard time interval between the transmission of subsequent symbols, thus avoiding the need for a complex equalizer at the receiver. The guard time allows delayed copies of each symbol, arriving at the receiver after the intended signal, to die out before the succeeding symbol is received.
There are several different schemes of multiple sub-carrier modulation possible for communications between the mobile units and the base stations. The most simple case is where each mobile unit has a different but fixed and contiguous block of sub-carriers. Other possibilities are to use hopping of the sub-carriers, or to use a random (non-contiguous) set of sub-carriers. Furthermore, it is also possible to use multiple sub-carrier CDMA, where several mobile units can share the same block of sub-carriers for transmitting respective data signals using different sub-carrier encoding and/or spreading codes (CDMA codes). As such, multiple sub-carrier CDMA refers to broadband multiple sub-carrier modulation with encoding and/or spreading of the data symbol on the sub-carriers using CDMA codes, thereby allowing multiple access to the same sets of sub-carriers.
A soft handover system for a multiple sub-carrier system is not known, but a soft handover system is needed to enable enhanced performance for multiple sub-carrier communications systems.